1. Field of the Invention
This invention relates to a projection type image-display apparatus.
2. Related Background Art
With the advent of the so-called multimedia age in recent years, image-display apparatus are found in various scenes of life. Particularly, since projection type image-display apparatus are adapted to be used with a large display screen, they are currently very popularly used in various applications including presentations such as sales presentations, for which those of the front projection type may be adequate, and home theaters, for which those of the rear projection type are suited.
While CRTs are conventionally used as optical modulators of such projectors, liquid crystal panels and DMDs (digital mirror devices) adapted to modulate the quantity of light by modifying the angle of the mirror are currently in the main stream because they can meet the requirement of enhanced brightness and high definition. For example, Japanese Patent Application Laid-open No. 10-78550 describes a DMD. A projection type image-display apparatus comprising a liquid crystal panel or a DMD is so designed that the liquid crystal panel or the DMD is illuminated with light from a light source and an image is formed on the display screen by transmitted or reflected light coming to it by way of a projection optical system.
Meanwhile, projection type image-display apparatus have not got to the level of direct-viewing type CRT image-display apparatus in terms of image quality particularly from the viewpoint of the texture of the displayed image. The expression of xe2x80x9cimage quality (texture)xe2x80x9d as used herein refers to dynamic range (contrast and gradation of images). Direct-viewing type CRT image-display apparatus realize a dynamic range higher than 10,000:1 for black and white images on the display screen.
On the other hand the dynamic range of a projection type image-display apparatus is principally determined by the performance of the optical modulator and approximately 300 to 400:1 in the case where a liquid crystal panel is used and approximately 600 to 800:1 in the case where a DMD is employed. Such a low dynamic range is partly due to the fact that, when a transmission type liquid crystal panel is used, leaking light is projected onto the screen showing one or more than one black areas and, when a reflection type liquid crystal screen or a DMD is used, scattered light is projected onto the screen also showing one or more than black areas.
In view of this problem, for example, Japanese Patent Application Laid-open No. 7-84553 proposes a projection type image-display apparatus comprising a scattering type liquid crystal panel. According to the above identified patent document, the projection optical system is provided with a large diameter aperture and a small diameter aperture, which are selectively used depending on the type of the input image to improve the dynamic range of the displayed image.
However, while an apparatus described in the above Japanese Patent Application Laid-open No. 7-84553 can display an image with an improved dynamic range, the brightness of the image is low to make the image poorly sharp.
Under the above identified circumstances, it is therefore the object of the present invention to provide a projection type image-display apparatus that can show a sharp image with a high dynamic range or a high image quality.
According to the invention, the above object is achieved by providing a projection type image-display apparatus comprising:
an optical modulator for displaying a gradational image by controlling the condition of transmission or reflection of light;
a lighting unit for irradiating said optical modulator with light;
a projection optical system for projecting the light emitted to and transmitted or reflected by said optical modulator;
a write signal processing means for modulating a write signal transmitted to said optical modulator;
a projected light quantity control means for controlling the quantity of light transmitted or reflected by said optical modulator; and
a control signal generation means for controlling said write signal processing means and said projected light quantity control means;
said control signal generation means being adapted to control the operation of controlling the quantity of projected light of said projected light quantity control means and that of modulating the write signal of said write signal processing means on the basis of the brightness level of the input image signal.